Electronic shearography apparatus for producing animation of shearogram images

ABSTRACT

The invention relates to an apparatus for performing electronic shearography on a test object, especially a tire or retread tire. The apparatus uses a laser light source to illuminate the test object. An optical element through which electromagnetic radiation is reflected from the test object is transmitted and forms a random interference image. The random interference image is electronically processed to provide a video animation of the effects of stress on the test object.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This patent application is a continuation of U.S. patent applicationSer. No. 09/334,311 filed Jun. 16, 1999 now U.S. Pat. No. 6,219,143, thedisclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of nondestructivetesting. Specifically, the present invention relates to the technique ofelectronic shearography. More specifically, the present inventionrelates to the use of electronic shearography to detect defects invehicle tires by animating shearograms produced while the tires undergoa varying stress continuum.

BACKGROUND OF THE INVENTION

The technique of shearing interferometry, or shearography involves theinterference of two laterally displaced images of the same object toform an interference image. Conventional shearographic methods requirethat a first interference image (or baseline image) be taken while theobject is in an unstressed or first stressed condition, and anotherinterference image be taken while the object is in a second stressedcondition. Comparison of these two interference images (preferably bymethods of image subtraction) reveals information about the strainconcentrations and hence the integrity of the object in a single imagecalled a shearogram. In particular, shearography has been shown to beuseful to detect strain concentrations and hence defects in vehicletires, especially retread vehicle tires.

In conventional electronic shearography, interference images are storedin a computer memory and are compared electronically to produce singlestatic shearograms. Because all the data are processed electronically,the results of the analysis can be viewed in “real time”. “Real time”,as used in the prior art, refers to the ability to view the shearogramnearly instantaneously after the second interference image has beentaken.

An apparatus and method for performing electronic shearography isdescribed in U.S. Pat. No. 4,887,899 issued to Hung. The apparatusdescribed in the cited patent produces an interference image by passinglight, reflected from the test object, through a birefringent materialand a polarizer. The birefringent material, which can be a calcitecrystal splits a light ray, reflected from the object, into two rays,and the polarizer makes it possible for light rays reflected from a pairof points to interfere with each other. Thus, each point on the objectgenerates two rays, and the result is an interference image formed bythe optical interference of two laterally displaced images of the sameobject.

Prior to the developments disclosed in the Hung patent, the spatialfrequency of the interference image produced in shearographic analysiswas relatively high requiring the use of high resolution photographicfilm to record a useful interference image. The development disclosed inthe Hung patent produces an interference image with a relatively lowspatial frequency because the effective angles between the interferingrays are small. Therefore, the interference images can be recorded by avideo camera, a video camera normally having much less resolvingcapability than a high density or high resolution photographic film. Bystoring an interference image of the object in its initial, unstressedcondition, and by comparing that interference image, virtuallyinstantaneously, by computer with another interference image taken undera different level of stress, a “real time” image or shearogram of theresultant strains on the object can be observed. Each point on theactual interference image is generated by the interference of lightemanating from a pair of distinct points on the object. Therefore, eachpixel of the video camera is illuminated by light reflected from thosetwo points. If the overall illumination remains constant, then anyvariations in the pixel intensity, in the interference image, will bedue only to changes in the phase relationship of the two points oflight.

When the initial video image of the interference image is stored, aninitial intensity for each pixel is recorded, as described above. Ifdifferential deformations occur in the object, such deformations willcause changes in the subsequent interference image. In particular, theintensity of a given pixel will change according to change in the phaserelationship between the two rays of light, reflected from the twopoints on the object, which illuminate the pixel. The phase differencescan be either positive changes, causing the pixel to become brighter ornegative changes, causing the pixel to become darker. Whether the pixelbecomes brighter or darker depends on the initial phase relationship andthe direction of the change of phase. Due to the cyclic nature of phaseinterferences, as the deformation of the object continually increases,the intensity at a given pixel may pass through a complete cycle. Thatis, the intensity of the pixel might increase to a maximum (positive)difference, then return to the original intensity, and then continue toa maximum (negative) difference, and so on.

In systems of the prior art, a single shearogram is derived from twosingle static interference images taken at two distinct stress levels.The single shearogram is then viewed by an operator for analysis ifmultiple shearograms are taken, the analysis is done one shearogram at atime. Thus, the operator attendance time, required to perform a thoroughstress analysis, is substantial. Further, a single shearogram mayfalsely show light features that appear to be defects (referred to as“false positives”). These “false positives” are caused by differentreflective characteristics on the surface of the test object and appearas defects when a static shearogram is viewed. Further still, in astatic shearogram some real defects may be “washed out” and thus notvisible (referred to as “false negatives”), at certain (particularlyhigh) stress levels. These “washed out” effects are caused byshearographic fringe lines that are not spatially separated enough to bevisibly distinguishable and therefore appear to be aberrational lighteffects rather than real defects in the test object. Thus, a singlestatic shearogram may contain inaccurate information with regards to thedefects actually present. Furthermore, an operator having to analyze alarge number of shearograms requires a large amount of operatorattendance time.

There is a need and desire for an improved method of presentation ofshearographic images that provide advantages over the prior art. Thereis also a need and desire for a method of presenting shearographicimages that provide improved accuracy, shorter attendance times by anoperator, and shorter overall cycle times for a test object. Further,there is a need and desire for a method of presenting shearographicimages that reduce the undesirable effects of false negatives bypreventing “wash out” of larger defects at high stress levels. Furtherstill, there is a need and desire for a method of presentingshearographic images that allows real defects to be distinguished overlight features that otherwise may be confused as defects, therebyminimizing false positives.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for performing electronicshearography on a test object. The apparatus includes a source ofcoherent electromagnetic radiation for illuminating the test object, andan optical element through which electromagnetic radiation reflectedfrom the test object is transmitted forming an interference image. Adetector converts the interference image into an electrical signalrepresentative of the interference image. An animation device is coupledto the detector. The animation device receives the electrical signalrepresentative of the interference image. The animation device retainsimage information derived from the electrical signals representative ofthe interference image at a predetermined frame rate. The animationdevice compares the retained interference image information with abaseline interference image to produce a shearogram image, and theanimation device is adapted to play a series of sequential shearogramimages. A display device is coupled to the animation device, providingvisualization of the sequential shearogram images.

The present invention further relates to a method of analyzing a testobject. The method includes directing coherent electromagnetic radiationonto a test object, providing electromagnetic radiation reflected fromthe test object to an optical shearing device, the optical shearingdevice creating an interference image, and directing the interferenceimage, emerging from the shearing device, onto a detector. The methodfurther includes capturing an electrical signal, communicated from thedetector, in a capture device, the electrical signal beingrepresentative of the interference image, storing interference imageinformation in a memory device communicated from the capture device andcomparing interference image information stored in the memory device, toa stored interference image to produce a shearogram image. The methodstill further includes repeating the aforementioned steps at varyingstress levels and displaying shearogram image information at a framerate.

The present invention still further relates to an apparatus forperforming electronic shearography on a tire undergoing varying statesof stress. The apparatus includes a source of coherent electromagneticradiation for illuminating the tire, a birefringent material throughwhich electromagnetic radiation reflected from the tire is transmitted,and a polarizer through which electromagnetic radiation, emerging fromthe birefringent material, is transmitted, the birefringent material andthe polarizer cooperating to form an interference image. The apparatusalso includes a video camera, the video camera converting theinterference image to an electrical signal and a video capture circuitcoupled to the video camera, the capture circuit receiving theelectrical signal from the camera, the electrical signal beingrepresentative of the interference image, the capture circuit retainingimage information derived from the electrical signals representative ofthe interference image at a frame rate. Further, the apparatus includesa computer coupled to the capture circuit, the computer adapted tocompare sequential interference images retained by the capture circuitto a baseline image to produce a shearogram image, the computer adaptedto play the sequential shearogram images, and the computer including adisplay device coupled to the computer providing visualization of thesequential shearogram images and a memory device, coupled to thecomputer, the memory device being adapted to store the interferenceimage information retained by the capture circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereafter be described with reference to theaccompanying drawings, wherein like reference numerals denote likeelements in the various drawings, and;

FIG. 1 is a schematic block diagram of a shearographic imaging system;

FIG. 2 is a schematic diagram of a shearographic imaging system showinga cross-section of a tire as the test object;

FIG. 3 is a schematic diagram of a shearographic camera at two differentorientations relative to the tire; and

FIGS. 4 and 5 are a graphical representation of the deformation of atest object, showing the corresponding shearographic fringe patternproduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention utilizes basic concepts of electronicshearography. More details of electronic shearography are given in U.S.Pat. No. 4,887,899, the disclosure of which is incorporated by referenceherein.

Referring now to FIG. 1, a schematic block diagram of an arrangement forpracticing electronic shearography is depicted. Coherent electromagneticradiation or coherent light is produced by a laser 10, the laser lightbeing directed through a fiberoptic cable 15 (or alternatively directedby a mirror or a set of mirrors or provided directly) to a beam expanderor illuminator 20. Beam expander 20 directs the coherent light onto atest object 25. The surface of test object 25 is illuminated andreflects light into a shearography camera 30. Shearography camera 30includes an optical element 35, a lens 40 for focusing the light, and adetector 45. Optical element 35 may be a birefringent material and apolarizer, the birefringent material being a calcite material such as aWallestein prism. The optical element is however not limited to abirefringent material and a polarizer, other elements such as adefraction grating, a Mickelson mirror, or an appropriate wave plate maybe applied. Further, optical element 35 may contain other optics, suchas, but not limited to a quarter-wave plate. Detector 45 may be atraditional video camera, a digital video camera, a charge coupleddevice (CCD), or other photo sensitive detection equipment.

The output of detector 45 is coupled to an animation device such as acomputer 50. Computer 50 includes a video capture circuit 55, a centralprocessing unit 60, and a memory 65. Alternatively, computer 50 mayinclude a logical extractor that is configured to extract shearographicimages from memory in a predetermined manner. The logical extractor maybe embodied in hardware or alternatively in software within computer 50.Video capture circuit 55 may be a dedicated video card or a framegrabber preferably capable of capturing entire video images at a rate ofat least 15 frames per second. However, video capture circuit 55 may becapable of capturing video images at any suitable rate. Centralprocessing unit 60 may be any of a number of conventionalmicroprocessors or a dedicated microprocessor device. Detector 45 iscoupled to central processing unit 60, central processing unit 60 beingcoupled to video capture circuit 55 and memory device 65. Centralprocessing unit 60 is further coupled to a display unit 70, which may bea CRT (cathode ray tube) display, an LCD (liquid crystal display), orthe like.

In operation, coherent light emanating from beam expander 20 isreflected from test object 25. Optical element 35 collects the reflectedlight from object 25 causing an interference image to be created. Theinterference image is focused on detector 45 through lens 40.Conventionally, a first interference image is taken while test object 25is in a first stressed condition, and a second interference image istaken with object 25 in a second stressed condition. The twointerference images are then compared by a process of subtracting oneimage from the other and the shearogram is created and displayed on amonitor.

In the present invention, test object 25 undergoes a sequence of orcontinuum of varying stress levels. Detector 45 continuously capturesthe interference image from optical element 35 and communicates theinterference image to computer 50, during the stress cycle. Capturecircuit 55 electronically captures entire interference images at a rateof at least 15 frames per second. Capture circuit 55 communicates theinterference images to central processing unit 60. Central processingunit 60 compares the interference image to a baseline interference imageof the object in the unstressed or near unstressed state (oralternatively any chosen stress state), by a process of subtracting oneinterference image from the baseline interference image, thereby forminga shearogram. Each shearogram image is simultaneously displayed ondisplay unit 70 and stored in memory device 65. After the series ofvarying stress levels has been completed, microprocessor 60 (oralternatively a logical extractor) recalls the sequence of shearogramimages captured by capture circuit 55 and replays them in sequence ondisplay unit 70. The sequential display of these shearogram images, at arate of at least 15 frames per second, produces a shearographicanimation of the shearograms produced during or after stressing of testobject 25.

Test object 25 may be a relatively large object, such as a tire 200, asdepicted in FIG. 2. A shearographic camera 230 that is rotatable withinthe inside of the bead 202 of tire 200 is depicted in FIG. 2.(Alternatively, tire 200 may be rotated and camera 230 may bestationary.) Shearographic camera 230 includes a laser 235 producing acoherent beam of light to illuminate the inside of tire 200.Shearographic camera 235 is further coupled to a computer 240 having adisplay 245, computer 240 and display 245 being used for dataacquisition and animation of the resultant shearographic images.

When used for detection of defects in tires or retread tires,shearographic imaging camera 230 may be positioned inside the tiredepicted as position A in FIG. 3 or outside the tire as depicted in FIG.3 by position B. Having shearographic camera 230 in position A allowsfor detection of defects in the tread area of tire 200. Havingshearographic camera 230 in position B provides for examination of thebead area and side wall area of tire 200.

Referring back to FIG. 2, in operation, shearographic camera 230 andtire 200 may be placed into a vacuum chamber capable of subjecting tire200 to a vacuum producing stresses on tire 200 by producing a positivepressure (relative to the pressure inside the vacuum chamber) in voidswithin tire 200 causing a bulge 250. Referring to FIG. 4, the bulge maybe caused by a defect 260, defect 260 possibly being but not limited toa delamination between two layers of the tire or a void in the moldedmaterial. When subjected to a vacuum, bulge 250 appears because ofpositive pressure within the void space of bond 260. The graph of FIG. 4depicts the slope of bulge 250 by line 270. The graph of FIG. 4 furtherdepicts a fringe pattern, including groups of rings 280 and 290,produced by the differencing of two optical interference images producedby shearographic camera 230. Fringe patterns 280 and 290 of a shearogramimage is produced by computer 240 (by the method of differencing or byany other image resolving technique) appear as a set of roughlyconcentric, substantially circular fringe lines corresponding to slope270 of bulge 250. Fringe patterns 280 and 290 are a contour mapping ofthe absolute value of slope 270 of bulge 250. Therefore, because bulge250 is substantially symmetric, fringe patterns 280 and 290 appear to bemirror images of each other.

Referring back to FIG. 2, in operation, shearographic camera 230 takes aseries of interference images that are communicated to computer 240while tire 200 undergoes varying vacuum or stress cycle. In a preferredembodiment tire 200 undergoes a depressurization cycle and then apressurization cycle to return the tire to an unstressed state. Becausethe field of view of shearographic camera 230 is limited by the field ofview of the optical elements and by the size of the tire, a tire must besectioned into a number of sectors ranging from four to twelve, or more.In an exemplary embodiment, tire 200 is sectioned into nine differentsectors. Shearographic camera 230 therefore views an area correspondingto 40° of arc of tire 200. After the depressurization and pressurizationcycle, camera 230 is rotated to the next sector, there thedepressurization and pressurization cycle is repeated. Computer 240continues to collect data and may, in a preferred embodiment,simultaneously display data on display 245 throughout the entirety ofthe nine sector cycle. The shearograms are generated and displayed at arate such that they appear to be animated.

Referring now to FIG. 5, a display 300 is depicted, the display beingdivided into nine different sectors, each sector 310 corresponding to anapproximate 40° arc of the inside of a tire. Alternatively, however,each sector 310 could correspond to any specific field of view, of atire, for a shearographic camera, such as shearographic camera 230.Computer 240 as depicted in FIG. 2, which may be connected to display300, is capable of displaying a plurality of animations simultaneouslyas depicted in FIG. 5. FIG. 5 depicts a static screen shot of a typicaldisplay, however, display 300 actually shows animations or sequentialimaging of shearogram images produced by computer 240 at a rateproviding an animated effect and in a preferred embodiment at a rate of30 frames per second. A display having multiple animation windows orscreen sectors provides the clear advantage that an operator may observethe animations simultaneously looking for the appearance of indicationsof deformations due to defects. This simultaneous observation permitsless attendance time by an operator, therefore providing substantialtime savings without substantial loss of accuracy. Capturing andproviding animation preferably at 30 frames per second (or alternativelyany suitable animation rate) provides animations that are sufficientlysmooth to be useful to an operator.

The advantages of animating the sequence of images is that animationimproves accuracy in the detection of defects. Light effects that wouldappear as “false positives” in a static shearogram are not manifested asdefects when animated, due to the absence of apparent motion induced bythe animation. A fringe pattern caused by a real defect will tend to“grow” or “shrink” and the intensity of fringe lines will appear tocycle during the animation, due to the continually changing stress stateon the test object. Furthermore, real defects that may be “washed out”in a static shearogram or even in an integration of multipleshearographic images, become apparent with animation of theshearographic images.

Animation of the shearographic images allows visualization of defects ata multiplicity of stress states, some of the stress states may not causethe “washed out” effect and further the apparent motion created byanimation of the images manifests a real defect as opposed to the lighteffect. Animation of the shearograms goes through a substantialcontinuity of stress states, therefore defects that may not be presentat two chosen stress states become apparent in the animation. Theseadvantages in animation of the shearographic images provide betteraccuracy in detecting defects and provides for shorter analysis times byan operator.

It has been recognized that a number of signal processing techniques,such as, but not limited to the use of fuzzy logic, neural networks,artificial intelligence, and pattern recognition techniques, may beapplied to perform automatic defect identification. However, systemssuch as this tend to be inherently complex and substantially costly.Therefore, retaining a human operator, but cutting down on theoperators' required attendance time by providing the operator withnumerous simultaneous animations, has the effect of providingsubstantial cost savings.

Although animation of shearographic images may be preferable at a rateof at least 15 frames per second, it should be noted that frame rates ofless than 15 frames per second may also be used effectively, however theanimation may appear discretized as compared to an animation running atleast 15 frames per second. Further, it should be appreciated that framerates of more than 30 frames per second may be advantageous in specificapplications and may become simpler to implement as microprocessor andvideo capture technology is improved.

It should be appreciated that although a differencing approach toproducing each shearogram is described above, the methods andapparatuses disclosed may be applied to different image resolvingtechniques, including but not limited to continuous integration.Continuous integration describes the process of taking a firstinterference image and subtracting a second interference image toproduce a first shearogram. A third interference image is taken andsubtracted from the first shearogram to produce a second shearogram. Afourth interference image is then taken and subtracted from the secondshearogram to produce a third shearogram. This sequence is continuedthroughout the testing cycle. The continuous integration technique andother techniques known to those of ordinary skill in the art, lendthemselves to the animation techniques disclosed above and can beapplied thereto without departing from the spirit and scope of thepresent invention.

The process and apparatus described above should be appreciated tooptimize a number of competing factors associated with shearographicimaging, especially as applied to the testing for defects in retreadtires (although clearly not limited to this application). Thesecompeting factors include, but are not limited to, maximizing data,maximizing accuracy, minimizing operator attendance time, availablelight wavelengths, object size, equipment costs, and optical field ofview. By animating shearograms in a plurality of sectors on a displayscreen, a number of these competing factors are optimized.

It is understood that, while the detailed drawings and examples givendescribe preferred exemplary embodiments of the present, they are forpurposes of illustration only. The method and apparatus of the inventionis not limited to the precise details and conditions disclosed. Forexample, the invention is not limited to the specific frame rates atwhich shearographic images are captured or displayed. Further, thenumber of sectors of the test object is completely variable and, theobject being tested may be any of a number of test objects. Stillfurther, the method by which the test object is placed under stress maybe any of a number of techniques. Still further, other optical systemsthat produce interference images, other than shearographic camera 30,may be applied to produce shearograms. Various changes may be made tothe details disclosed without departing from the spirit of theinvention, which is defined by the following claims.

What is claimed is:
 1. An apparatus for performing electronicshearography on a test object comprising: a shearography camera fortaking an interference image of the test object, an image processorcoupled to the shearography camera for receiving a plurality ofsequential interference images from the shearography camera, producing aplurality of sequential shearogram images of the test object from theinterference images and animating the sequential shearogram images torepresent dynamically changing stress states on the test object, and adisplay coupled to the image processor for providing visualization ofthe animation of the sequential shearogram images.
 2. The apparatusaccording to claim 1 wherein the image processor further includes amemory device for storing the shearogram images.
 3. The apparatusaccording to claim 2 wherein the image processor animates the shearogramimages stored in the memory device.
 4. The apparatus according to claim1 wherein image processor receives interference images from theshearography camera at a frame rate of at least fifteen frames persecond.
 5. The apparatus according to claim 1 wherein the imageprocessor animates the shearogram images at an animation rate of atleast fifteen frames per second.
 6. The apparatus according to claim 1wherein the image processor substantially simultaneously animatesmultiple shearogram image sequences representative of different sectionsof the test object.
 7. The apparatus according to claim 1 wherein theimage processor is a computer.
 8. A method for analyzing a test objectcomprising: (a) taking an interference image of a test object, (b)comparing the interference image with a baseline interference image toproduce a shearogram image, (c) repeating steps (a) and (b) at varyingstress levels to produce a plurality of shearogram images, and (d)displaying the plurality of shearogram images at a frame rate fastenough to generate an animation representative of dynamically changingstress states on the test object.
 9. The method according to claim 8further including the step of storing the shearogram images.
 10. Themethod according to claim 8 wherein the frame rate is at least fifteenframes per second.
 11. The method according to claim 8 furthercomprising simultaneously displaying multiple shearogram image sequencesrepresentative of different sections of the test object.